Wafer bonding is commonly employed for wafer level 3-D packaging. For example, two wafers are bonded together prior to dicing. After the wafers are bonded to form a wafer stack, the wafer stack is subjected to wafer bumping processing which forms external connections. Conventional wafer bumping processing has been found to cause damage to the wafer stack. For example, chemicals used in wafer bumping processing may damage interconnections between the wafers. Such damages adversely affect reliability and yield.
From the foregoing discussion, it is desirable to provide a wafer stack that avoids damage caused by, for example, wafer bumping processing.